flake.lock: Update

Flake lock file updates:

• Updated input 'flake-utils':
    'github:numtide/flake-utils/f9e7cf818399d17d347f847525c5a5a8032e4e44' (2023-08-23)
  → 'github:numtide/flake-utils/ff7b65b44d01cf9ba6a71320833626af21126384' (2023-09-12)
• Updated input 'neovim-nightly':
    'github:neovim/neovim/aab06edc63c3dfbd99e0c803c6834bf771318c42?dir=contrib' (2023-09-11)
  → 'github:neovim/neovim/1f551e068f728ff38bd7fdcfa3a6daf362bab9da?dir=contrib' (2023-09-12)
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/78058d810644f5ed276804ce7ea9e82d92bee293' (2023-09-10)
  → 'github:nixos/nixpkgs/1ee50a29288f768c55211963be8040671814986d' (2023-09-12)